On 1998/03/11 Frank LeFever <flefever at ix.netcom.com> wrote
> In the beginning, in very primitive ancestral organisms, a
sensory cell communicated directly to a motor cell, so that
a stimulus (e.g. a touch) produced a response (e.g. movement
away) ...
I came to the same conclusion about the cerebrum starting from
"smell cells" after reading Buttler & Hodos [1].
I'd like to expand on Frank's tale. Any details to fill in the
gaps (or point out where I'm completely wrong) would be
appreciated.
-----
In the beginning, single cells floated aimlessly in a warm sea.
If they bumped into something that was "food," they ate. If they
didn't before they used up their chemical energy, or if they
bumped into something that ate them -- too bad.
Later, some cells developed cilia, little hair-like extensions on
their outer membranes that allowed them a certain amount of
purposeful locomotion. (Humans have cilia on the inner walls of
the intestines to move food along, and something like interlocked
cilia form muscles.) It helped if the cells could detect the
waste products of their prey and thus direct their motion towards
it.
Later, cells such as slime mold, formed colonies. The cells in
the leading edge of the slime mold detected "food" and emitted
what would be the first neurotransmitter out their back ends to
tell their fellow slime mold cells, "Food -- this way!"
Now these cells didn't do anything to avoid being prey
themselves; they only "hoped" they could eat faster and reproduce
faster than they were eaten.
Now comes specialization. Some cells dropped their cilia and
concentrated on the business of smelling, and let cells behind
them push them along. Some of the cells in the back end got
tired of being eaten, and developed their own neurotransmitters
to tell their fellows up front, "Move it up there! Somethings
about to eat me!"
As things got more complicated, cells in the middle developed the
ability to detect pressure, which might indicate prey/predator
(remember, everything is still aquatic at this point). Of
course, specialized digestive, circulatory, and protective
[shell] systems also developed, but I don't want to dwell on that
here. All the "important" neural cells ended up in the center of
what now is an animal, and let the "lesser" cells protect and
feed them (sort of like present-day governments).
One can imagine such an animal might look like a shrimp. The
smell cells became more diverse and specialized, to try to "look
ahead" and determine what subtle combinations of chemicals might
mean food.
At first, "eyes" -- or maybe just one, the pineal eye on top of
the animal's head -- were only used to detect light or dark.
However, now the animal was large enough that it could get hurt
if it bumped into a rock too hard, so other eyes developed at the
side of the "head." In vertebrates, these eyes developed from
brain tissue, which is why our retinas are "inside out" from any
logical design: light has to pass through the thickness of the
retina to reach the light-sensitive cells.
Further increases in complexity meant that the original eyes
couldn't interpret everything, so they grew out on long stalks
(which still were inside the skin of the animal). Some of the
original "smell cells" specialized to interpret signals from the
eyes. At the same time, cells that measured pressure on the
outside of the animal also specialized.
The net result is that what was the original animal's brain,
which decided which of the actions requested by is sensory system
from head to tail should be acted upon, ended up as the thalamus,
and the original eyes ended up as the lateral geniculate nuclei
on the sides of the thalamus. The somosensory parts, which took
over memory-generating functions, became the hippocampus, which
generated more specialized "smell-cell-like" cells that became
the bulk of our cerebral cortex. Finally, the original tail
cells (which still specialized in saying "ouch") became the
amygdala, which is the brain's center for fear.
If you don't believe all this, look at a picture of the lymbic
system [2], and you will see the original blue "shrimp," which is
my candidate for the homunculus. :)
(I'm sure there's a grain or truth somewhere in this tale.)
Patrick Spangler
[1] Comparative Vertibrate Neuroanatomy, Evolution & Adaptation
by Ann B. Buttler & William Hodos [Wiley-Liss, New York (1996)
ISBN 0-471-88889-3]
[2] http://www.harbrace.com/psych/rathus/
instructor/resources/oht/MT03_15.html